[78.18] Probing the Triggering & Fueling Mechanisms for Quasars

Supermassive black holes appear to be a ubiquitous component
of galaxy bulges, yet only a small fraction of these are
strongly accreting. Why? What is responsible for triggering
the fueling of the black holes to become quasars?
Conventional wisdom holds that agents external to the
quasar's host galaxy, such as nearby companions or a large
cluster environment, may play a significant role. However,
recent results, including our own, indicate that the
situation may be more complicated. We find little evidence,
based on R-band imaging, for large cluster-scale
environments around either radio-loud or radio-quiet
quasars.

Our sample consists of 33 quasar fields drawn from the
optically selected Large Bright Quasar Survey (LBQS). Most
of the quasars in our study lie in the redshift range 0.4 <
z < 0.5, though about 1/3 have z > 0.6. In general, the
environments are consistent with those of field galaxies.
Only two quasars appear to lie in Abell class 0 galaxy
clusters, and the error bars on one of these are large.
Similar results hold for a subsample of 16 LBQS quasars with
z < 0.5 imaged with HST (Finn et al. 2001, ApJ, 557, 578).

Smaller scale environments may exist around the quasar, and
in fact there are tantalizing indications of these,
including possible galaxy groups and companions to the
quasar host galaxies. However, they are statistically
undetectable in single-band images against the omnipresent
fog of field galaxies. We have begun spectroscopic follow-up
on about half of the fields to investigate whether sparse
environments are present and what role they play in
triggering quasar activity.

The first two authors gratefully acknowledge support of NSF
Astronomy & Astrophysics Postdoctoral Fellowships.